In a wireless communications system such as a mobile cellular communications system, a wireless local area network (Wireless Local Area Network, WLAN), and fixed wireless access (Fixed Wireless Access, FWA), a communications node such as a base station (Base Station, BS), an access point (Access Point, AP), a relay station (Relay Station, RS), or user equipment (User Equipment, UE) usually has capabilities of sending a signal of the communications node and receiving a signal of another communications node. Because a wireless signal is greatly attenuated on a wireless channel, compared with a signal sent by the communications node, a signal coming from a communications transmit end is very weak when arriving at a receive end. For example, a difference between a power for receiving a signal and a power for sending a signal by a communications node in a mobile cellular communications system can reach 80 dB to 120 dB or can be even larger. Therefore, to avoid interference (such interference is referred to as self-interference, Self-interference) to a received signal of a communications node from a sending signal of the communications node, transmission and reception of a wireless signal are distinguished by using different frequency bands or time periods. For example, in frequency division duplex (Frequency Division Duplex, FDD), communications in transmission and reception are performed by using different frequency bands separated by a certain guard band. In time division duplex (Time Division Duplex, TDD), communications in transmission and reception are performed by using different time periods separated by a certain guard interval. The guard band in the FDD system and the guard interval in the TDD system are both for the purpose of ensuring that reception and transmission are thoroughly separated, thereby avoiding interference to reception from transmission.
In the wireless full duplex technology, reception and transmission operations can be simultaneously performed on a same wireless channel. Theoretically, the spectral efficiency of the wireless full duplex technology is twice as high as that of the FDD technology or the TDD technology. However, because there is no guard band or guard interval, a transmitted signal of a communications node that supports wireless full duplex may result in self-interference to a received signal of the communications node, causing that the communications node cannot correctly receive a wanted signal. A self-interference signal includes a near-field reflected self-interference signal on a near-field reflection channel and a far-field reflected self-interference signal on a far-field reflection channel. The near-field reflected self-interference signal typically corresponds to a near-field reflection path of 0.3 m to 60 m, and a multi-path transmission delay is 1 ns to 400 ns. Because propagation environments around a transceiving antenna change slightly, a delay of the near-field reflected self-interference signal changes slightly and slowly with time. The near-field reflected self-interference signal is self-interference that is the most difficult to cancel effectively in a wireless full duplex system, and reasons are as follows: Because a propagation distance of a near-field multi-path echo signal is relatively short, a propagation delay difference between multiple paths is very small, when a communication signal of a normal bandwidth (10 to 40 MHz) is used, the near-field reflected self-interference signal cannot be effectively recognized or reconstructed, and effective interference cancellation cannot be implemented. For example, a difference between delays generated when a radio frequency signal sent by a communications node reaches two reflectors whose straight-line propagation distances from the communications node have a difference of 3 meters is 20 ns, and it is very difficult to distinguish between two echoes having different multi-path delays. Due to a relatively large multi-path delay difference, a component of a far-field reflected self-interference signal can be recognized when a signal having a normal bandwidth is used, thereby implementing effective cancellation. Therefore, how to reduce a near-field reflected self-interference signal to improve use efficiency of the wireless full duplex system is a problem to be resolved.